I’m not saying that the position is winning for black, but black’s game is very comfortable. Your pieces are coordinated and both queenside and kingside attacks are possible. Plus, don’t downplay the fact that it’s very disruptive to the plans of many Nimzo-Larsen players go for. With best play white is equal, but they have to be moves that they usually don’t like to make. For instance, if you retreat the bishop to d6, white’s best move in some positions is often to try to trade it off with Ba3, but I have never seen an N-L player willingly part with that bishop.
Edit: to be fair, though, you're an IM and I stink. The opponents you face are much more likely to be adaptive to the needs of the position than the players that I face, who are much more likely to stick with their usual plans even if they aren't the best.
You can't really play it against most openings but you can definitely play it against the Nimzo-Larsen.
Why 5...c6? The d5 square needs no extra protection at this point.
Believe it or not, it's the #1 suggested move by Stockfish. Generally, you can play Bb4 to induce a weakness (and then usually retreat it to d6) and you open up the queen's path to the a-file. You can play c5 instead, but I prefer to keep white's dark square bishop sad. It's largely psychological because a lot of Nimzo-Larsen players rely upon the dark square bishop and if you can keep it out of the game you're making them play a game they don't want to play.
The only reason I can see is answering 6.c4 with 6...Bb4+, when the "natural reaction" 7.Nc3? loses to 7...Qa5.
But then, I do not see why white has to hurry pushing c2-c4.